Radio frame numbers or frame numbers, respectively are usually used in digital cellular systems for implementing and managing the synchronization or signaling between terminals and base stations. The prior art proposes a huge number of encoding methods used by various systems. Generally, for signaling purposes a lot of information needs to be transmitted between network (base station) and the terminals. Synchronization generally means that the mobile station detects (i.e. temporally correctly records) a frame structure, which is specified in the base station by a data-structuring rule, of the transmitted data stream in the received radio signal. Thereby the mobile station has to be enabled to decode the corresponding received radio frame structures.
For that, WCDMA uses several physical layer control channels needed for system operation but not necessarily visible for higher level operations. The following paragraphs shortly mention the physical channels used for system operations between terminals and mobile stations.
The Common Pilot Channel (CPICH) is an unmodulated code channel, which is scrambled with a cell-specific primary scrambling code and it is generally used for channel estimation. Another important physical channel is the Synchronization Channel (SCH) mainly needed for cell search. It consists of two channels, a primary and a secondary channel. Once the terminal has identified the secondary SCH-channel, it has obtained frame and slot configuration as well and also main cell information. For this purposes a unique system frame number (SFN) may be needed. For the sake of completeness the remaining channels with their corresponding abbreviations are listed below: Primary Common Control Physical Channel (Primary CCPCH), Secondary CCPCH, Random Access Channel (RACH) for signaling transmission, Acquisition Indicator Channel (AICH), Paging Indicator Channel (PICH) and physical channels for CPCH access procedure. Their functionality is for a skilled person in the telecommunication field clear and it is out of scope of the present invention.
The channels mentioned above are designed for bidirectional transmission of data between base station and user equipment (terminal). They use a 10 ms radio frame structure wherein 72 grouped frames form a super frame of 720 ms. Each frame consists of a system frame number used by several procedures that span more than one frame. Some physical layer procedures may require longer frame periods than 10 ms for correct definition. The radio frame number in WCDMA is very short and its purpose is to provide the radio link level synchronization information only. The encryption of radio frames requires using an extended virtual frame number, which is initialized at the start of a call with a separate and rather complicated method, which is not based on broadcasting the frame number, but on the virtual frame number establishment for each call. The present frame number in WCDMA is 7 bits long and it is used to enumerate 72 frames, which is needed in low-level synchronization.
The synchronization blocks in GSM contain the full frame number (FN), but the radio capacity use has been minimized so that the SCH channel is transmitted relatively infrequently. This makes the synchronization to the base stations very slow, especially in adjacent cell monitoring during a call. Both sides, mobile station and base station use internal counters for synchronization issues. Once the mobile station receives the FN it is able to actualize the entire internal counters so that a synchronic communication to the base station is provided. The radio capacity consumption increases if a larger frame number is needed.